Filter device

ABSTRACT

A filter device includes a first path, a second path, and a capacitor. The first path includes at least one ladder filter circuit and connects a first terminal and a second terminal. The at least one ladder filter circuit includes a parallel arm resonator connected to a ground terminal. The second path includes a grounded resonator and is connected in parallel with any of the at least one ladder filter circuit. One end of the capacitor is connected to the second path, and the other end of the capacitor is connected to a third path which connects the parallel arm resonator and the ground terminal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to Japanese PatentApplication No. 2019-169404 filed on Sep. 18, 2019. The entire contentsof this application are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a filter device.

2. Description of the Related Art

Hitherto, a filter device that allows signals of a specific frequencyband to pass therethrough is mounted on a wireless communicationapparatus, such as a cellular phone. For example, a filter device thatseparates signals of different frequency bands, such as a transmitsignal and a received signal, from each other is mounted.

Japanese Unexamined Patent Application Publication No. 2018-88678discloses a filter device using a ladder filter circuit. Morespecifically, in this filter device, a loop circuit is disposed inparallel with the ladder filter circuit, thereby improving the isolationcharacteristics of the filter device.

Typically, a parallel arm resonator in a ladder filter circuit generatesan attenuation pole by utilizing sub-resonance formed in aradio-frequency band of about several gigahertz, thereby contributing tothe attenuation of a radio-frequency signal. To adjust the frequencyband of a signal to be attenuated by sub-resonance, it is necessary tocontrol the attenuation pole generated by the sub-resonance. Forexample, it is desirable that a filter device used for Wi-Fi (registeredtrademark) communication is able to adjust the attenuation polegenerated by sub-resonance.

To shift the attenuation pole generated by sub-resonance to the lowerfrequency side, for example, the capacitance of a parallel arm resonatorof a ladder filter circuit may be increased. Increasing the capacitanceof a parallel arm resonator, however, makes it difficult to reduce thesize of the filter device.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide filter deviceswhich are each able to shift an attenuation pole generated bysub-resonance to a lower frequency side while achieving a small size ofthe filter device.

According to a preferred embodiment of the present invention, a filterdevice includes a first path, a second path, and a capacitor. The firstpath includes at least one ladder filter circuit and connects a firstterminal and a second terminal. The at least one ladder filter circuitincludes a parallel arm resonator connected to a ground terminal. Thesecond path includes a grounded resonator and is connected in parallelwith any of the at least one ladder filter circuit. One end of thecapacitor is connected to the second path, and the other end of thecapacitor is connected to a third path which connects the parallel armresonator and the ground terminal.

According to a preferred embodiment of the present invention, it ispossible to provide a filter device which is able to shift theattenuation pole generated by sub-resonance to the lower frequency sidewhile achieving a small size of the filter device.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a filter device according to a firstpreferred embodiment of the present invention.

FIG. 2 is a circuit diagram of a receive filter circuit included in thefilter device according to the first preferred embodiment of the presentinvention.

FIG. 3 is a top view partially illustrating the filter device accordingto the first preferred embodiment of the present invention.

FIG. 4 is a sectional view taken along line A-A′ in FIG. 3 .

FIG. 5 is a graph illustrating the isolation characteristics of thefilter device according to the first preferred embodiment of the presentinvention in a path from a transmit input terminal to a receive outputterminal.

FIG. 6 is a graph illustrating the attenuation characteristics of thefilter device according to the first preferred embodiment of the presentinvention in a path from the transmit input terminal to a second commonterminal.

FIG. 7 is a circuit diagram of a filter device according to a secondpreferred embodiment of the present invention.

FIG. 8 is a circuit diagram of a transmit filter circuit included in thefilter device according to the second preferred embodiment of thepresent invention.

FIG. 9 is a graph illustrating the isolation characteristics of thefilter device according to the second preferred embodiment of thepresent invention.

FIG. 10 is a circuit diagram of a filter device according to a thirdpreferred embodiment of the present invention.

FIG. 11 is a graph illustrating the isolation characteristics of thefilter device according to the third preferred embodiment of the presentinvention.

FIG. 12 is a circuit diagram of a filter device according to a firstmodified example of a preferred embodiment of the present invention.

FIG. 13 is a circuit diagram of a filter device according to a secondmodified example of a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described belowwith reference to the accompanying drawings. The same or similarelements are designated by the same reference numerals in the drawings.

1. First Preferred Embodiment 1.1. Circuit Diagram of Filter Device

FIG. 1 is a circuit diagram of a filter device 1 according to a firstpreferred embodiment of the present invention. FIG. 2 is a circuitdiagram of a receive filter circuit 14 included in the filter device 1of the first preferred embodiment.

The filter device 1 includes a transmit input terminal Tx, a receiveoutput terminal Rx, a first path 10, a second path 20, a second commonterminal 30, and a first capacitor C1, for example.

The first path 10 connects the transmit input terminal Tx and thereceive output terminal Rx and includes a transmit filter circuit 12 anda receive filter circuit 14.

A transmit signal output from a transmit circuit (not shown) is suppliedto the transmit filter circuit 12 via the transmit input terminal Tx.The transmit filter circuit 12 allows a transmit signal of apredetermined frequency band to pass therethrough and to be output to afirst common terminal 16, while it attenuates transmit signals of otherfrequency bands. The transmit signal passed through the transmit filtercircuit 12 is sent to the second common terminal 30 via the first commonterminal 16 and is further sent to a base station from the second commonterminal 30.

A received signal received from a base station via the second commonterminal 30 is supplied to the receive filter circuit 14 via the firstcommon terminal 16. The receive filter circuit 14 allows a receivedsignal of a predetermined frequency band to pass therethrough andattenuates received signals of other frequency bands. The receivedsignal passed through the receive filter circuit 14 is sent to a receivecircuit (not shown) via the receive output terminal Rx.

The filter device 1, which includes the transmit filter circuit 12 andthe receive filter circuit 14 as described above, thus defines andfunctions as a duplexer.

The transmit filter circuit 12 in the first preferred embodiment is aladder filter circuit including multiple resonators connected in serieswith each other and multiple resonators connected in parallel with eachother. More specifically, the transmit filter circuit 12 includes fourseries arm resonators S11 through S14 disposed on a series arm and threeparallel arm resonators P11 through P13 disposed on a parallel arm. Theparallel arm resonators P11 and P12 are connected to a ground terminalg11, while the parallel arm resonator P13 is connected to a groundterminal g12. The path which connects the ground terminal g11 and theparallel arm resonator P11, which is closest to the transmit inputterminal Tx among the parallel arm resonators P11 through P13, will bereferred to as a third path 24.

The number of series arm resonators and the number of parallel armresonators are not limited to the above-described numbers. Three or lessor five or more series arm resonators may be provided. Two or less orfour or more parallel arm resonators may be provided. Similarly, thenumbers of various resonators in the second and third preferredembodiments are not limited to any particular numbers.

Each of the series arm resonators S11 through S14 and the parallel armresonators P11 through P13 is not restricted to a particular type ofdevice, and may preferably be a surface acoustic wave (SAW) filter, apiezoelectric thin-film resonator filter, or a bulk acoustic wave (BAW)filter, for example. The resonators in the second and third preferredembodiments may include at least one of the above-described device, forexample.

The configuration of the receive filter circuit 14 will be describedbelow with reference to FIG. 2 . As shown in FIG. 2 , the receive filtercircuit 14 is a ladder filter circuit and includes two series armresonators S21 and S22, two parallel arm resonators P21 and P22, and alongitudinally coupled resonator unit 140. The parallel arm resonatorsP21 and P22 are connected to a ground terminal g21.

The longitudinally coupled resonator unit 140 includes two5-interdigital-transducer (5IDT) longitudinally coupled resonators andthree reflectors 144 a, 144 b, and 144 c. One 5IDT longitudinallycoupled resonator includes five IDT electrodes 142 a through 142 e,while the other 5IDT longitudinally coupled resonator includes IDTelectrodes 142 f through 142 j.

Referring to FIG. 1 , the second path 20 will be explained. The secondpath 20 is connected in parallel with the transmit filter circuit 12 ofthe first path 10. More specifically, the second path 20 is connected toa node between the transmit input terminal Tx and the series armresonator S11 and also to the first common terminal 16. The second path20 includes a second capacitor C2, a 2IDT longitudinally coupledresonator 22, and a third capacitor C3 in this order in which they arelocated closer to the transmit input terminal Tx. The 2IDTlongitudinally coupled resonator 22 is grounded.

The second path 20 generates a cancel signal having the same orsubstantially the same amplitude and the inverted phase with respect toan unwanted signal in the first path 10, and then combines this cancelsignal with the signal in the first path 10. As a result, the unwantedsignal in the first path 10 can be attenuated. The second and thirdcapacitors C2 and C3 in the second path 20 have a function of adjustingthe amplitude of a cancel signal.

The 2IDT longitudinally coupled resonator 22 includes two IDT electrodesand has a function of adjusting the phase of a signal input into the2IDT longitudinally coupled resonator 22. In the 2IDT longitudinallycoupled resonator 22, the IDT pitch (electrode finger pitch), thepolarity of IDTs, and the IDT gap are set so that the phase of astanding wave excited in each IDT electrode deviates from that inanother IDT electrode by about λ/2, for example. The 2IDT longitudinallycoupled resonator 22 shifts the phase of an input signal by about λ/2 soas to generate an output signal whose phase is inverted with respect tothe input signal and outputs the output signal. As a result, the secondpath 20 is able to generate a cancel signal whose phase is inverted withrespect to an unwanted signal in the first path 10. The frequency bandof a signal to be subjected to a phase shift in the 2IDT longitudinallycoupled resonator 22 can be changed by adjusting the resonant frequencyof the IDT electrodes, for example. The amount of phase shift “aboutλ/2” includes an allowance of the phase value which may change due tothe manufacturing variations, for example.

The first capacitor C1 is connected to the second path 20 and the thirdpath 24. More specifically, one end of the first capacitor C1 isconnected to a node between the second capacitor C2 and the 2IDTlongitudinally coupled resonator 22 on the second path 20, while theother end is connected to the third path 24. Inductance (not shown) isgenerated by wiring, for example, between the ground terminal g11 and anode between the third path 24 and the first capacitor C1.

1.2. Structure of Filter Device

The structure of the filter device 1 according to the first preferredembodiment will be described below with reference to FIGS. 3 and 4 .FIG. 3 is a top view partially illustrating the filter device 1. FIG. 4is a sectional view taken along long dashed dotted line A-A′ in FIG. 3 .

The filter device 1 includes a piezoelectric substrate 42. If SAWs areutilized, the piezoelectric substrate 42 may preferably be made of apiezoelectric material, such as lithium tantalate (LiTaO₃) or lithiumniobate (LiNbO₃), for example. If BAWs are utilized, the piezoelectricsubstrate 42 may preferably be made of a piezoelectric material, such asaluminum nitride (AlN), for example. The transmit filter circuit 12, thereceive filter circuit 14, and the second path 20, for example, areprovided on the piezoelectric substrate 42.

In FIG. 3 , the parallel arm resonator P11 and the ground terminal g11of the transmit filter circuit 12 and the 2IDT longitudinally coupledresonator 22 and the second capacitor C2 of the second path 20 areshown. The 2IDT longitudinally coupled resonator 22 and the secondcapacitor C2 are connected with each other by a first wiring 43. Thefirst wiring 43 defines a portion of the second path 20. The parallelarm resonator P11 and the ground terminal g11 are connected with eachother by a second wiring 45. The second wiring 45 defines a portion ofthe third path 24. The portion of the first wiring 43 indicated by thebroken lines in FIG. 3 passes between the piezoelectric substrate 42 andthe second wiring 45. That is, the first wiring 43 and the second wiring45 three-dimensionally intersect with each other.

FIG. 4 is a sectional view taken along long dashed dotted line A-A′ inFIG. 3 . As shown in FIG. 4 , five layers are provided in the sectionalview of the filter device 1. More specifically, the piezoelectricsubstrate 42, the first wiring 43, an intermediate layer 44, the secondwiring 45, and a surface layer 46 are provided from the bottom in thisorder.

The intermediate layer 44 is disposed in a space sandwiched between thefirst wiring 43 and the second wiring 45. The intermediate layer 44 isdefined by an insulator, and may preferably be a ceramic layer made of aceramic material, for example. The ceramic layer may preferably be aglass layer made of SiO₂, for example. The surface layer 46 maypreferably be made of SiN, for example. The surface layer 46 is providedon the second wiring 45 so as to protect the surface of the secondwiring 45.

In the first preferred embodiment, as a result of the first wiring 43and the second wiring 45 opposing each other, the first capacitor C1 isprovided. The capacitance of the first capacitor C1 can be adjusted bysuitably changing the area by which the first wiring 43 and the secondwiring 45 oppose each other, the distance between the first wiring 43and the second wiring 45, and the material or the shape of theintermediate layer 44, for example. Changing the capacitance of thefirst capacitor C1 can adjust the amplitude and the phase of a cancelsignal generated in the second path 20. That is, the first capacitor C1is one of the parameters for adjusting a cancel signal. As a result ofadjusting the capacitance of the first capacitor C1, a suitable cancelsignal can be generated without changing (increasing, for example) thecapacitance of the second capacitor C2.

1.3. Advantages

Advantages of the filter device 1 according to the first preferredembodiment will be discussed below with reference to FIGS. 5 and 6 .FIG. 5 is a graph illustrating the transmission characteristics (alsocalled the isolation characteristics) of the filter device 1 in a pathfrom the transmit input terminal Tx to the receive output terminal Rx.FIG. 6 is a graph illustrating the attenuation characteristics of thefilter device 1 in a path from the transmit input terminal Tx to thesecond common terminal 30.

In FIG. 5 , the transmit band is the pass band of the transmit filtercircuit 12, while the receive band is the pass band of the receivefilter circuit 14. In FIG. 5 , the solid line indicates the isolationcharacteristics of the filter device 1 according to the first preferredembodiment, while the broken line indicates the isolationcharacteristics of a filter device according to a comparative examplenot including the first capacitor C1.

As shown in FIG. 5 , the attenuation in the receive band of the filterdevice 1 of the first preferred embodiment is greater than that of thefilter device of the comparative example. That is, the attenuation inthe receive band of the transmit filter circuit 12 is increased. Thereason for this is as follows. As a result of adding the first capacitorC1, more parameters are provided to generate a cancel signal. Thisfacilitates the generation of a cancel signal having the same orsubstantially the same amplitude and the inverted phase with respect toan unwanted signal, compared with when the first capacitor C1 is notprovided. Thus, a signal input into the transmit filter circuit 12 isless likely to leak into the receive filter circuit 14 and to mix with asignal in the receive band. As a result, the reception sensitivity isless likely to be degraded.

In the transmit filter circuit 12, typically, a radio-frequency signalis attenuated by an attenuation pole generated by sub-resonance providedin a radio-frequency band of about several gigahertz. Sub-resonance isseries LC resonance generated by the capacitance of a parallel capacitorin the equivalent circuit of a SAW resonator and the inductance of anexternal inductor connected to the SAW resonator. In this example, theattenuation pole generated by sub-resonance is provided by thecapacitance of the parallel arm resonators P11 through P13 and theinductance produced by the wiring which connect the parallel armresonators P11 and P12 and the ground terminal g11 and the parallel armresonator P13 and the ground terminal g12. More specifically, forexample, the attenuation pole is generated by series LC resonanceprovided by the capacitance of the parallel arm resonator P11 and theinductance of the wiring which connects the parallel arm resonator P11and the ground terminal g11. Without the first capacitor C1, in order toshift this attenuation pole to the lower frequency side, it is necessaryto increase the capacitance of the parallel arm resonator P11 or theabove-described inductance. To increase the capacitance or theinductance, it is necessary to increase the size of the parallel armresonator or to increase the inductance provided in or on an inner layerof a package of the filter device. This makes it difficult to reduce thesize of the filter device.

In FIG. 6 , the solid line indicates the attenuation characteristics ofthe filter device 1 according to the first preferred embodiment, whilethe broken line indicates the attenuation characteristics of the filterdevice of the comparative example not including the first capacitor C1.As shown in FIG. 6 , the attenuation pole in the filter device 1 of thefirst preferred embodiment is positioned at the lower frequency sidethan that of the comparative example. That is, as a result of providingthe first capacitor C1, the attenuation pole generated by theabove-described series LC resonance is shifted to the lower frequencyside.

In this manner, in the first preferred embodiment, the attenuation polegenerated by sub-resonance provided in a radio-frequency band can beadjusted without increasing the size of the parallel arm resonator orincreasing the inductance. That is, in the first preferred embodiment,it is possible to shift the attenuation pole to the lower frequency sidewithout increasing the size of the filter device 1.

2. Second Preferred Embodiment

A second preferred embodiment of the present invention will be describedbelow with reference to FIGS. 7, 8, and 9 . The second preferredembodiment will be discussed mainly by referring to points differentfrom the first preferred embodiment while omitting the same or similarpoints as the first preferred embodiment.

2.1. Circuit Diagram of Filter Device

FIG. 7 is a circuit diagram of a filter device 2 according to the secondpreferred embodiment. FIG. 8 is a circuit diagram of a transmit filtercircuit 13 included in the filter device 2 of the second preferredembodiment. FIG. 9 is a graph illustrating the isolation characteristicsof the filter device 2.

The configuration of the filter device 2 will first be described belowwith reference to FIGS. 7 and 8 . As shown in FIG. 7 , the filter device2 includes a transmit input terminal Tx, a receive output terminal Rx, afirst path 50, a second path 52, a second common terminal 30, and afourth capacitor C4.

The first path 50 connects the transmit input terminal Tx and thereceive output terminal Rx and includes the transmit filter circuit 13and a receive filter circuit 15. As shown in FIG. 8 , the transmitfilter circuit 13 includes four series arm resonators S11 through S14,three parallel arm resonators P11 through P13, and two ground terminalsg11 and g12, as in the transmit filter circuit 12 of the first preferredembodiment. The transmit filter circuit 13 in the second preferredembodiment is different from the transmit filter circuit 12 in the firstpreferred embodiment in that the parallel arm resonator P11 locatedclosest to the transmit input terminal Tx among the parallel armresonators P11 through P13 is not connected to the second path 52 via acapacitor.

Referring to FIG. 7 , the configuration of the filter device 2 will bedescribed. The receive filter circuit 15 includes two series armresonators S21 and S22, two parallel arm resonators P21 and P22, and alongitudinally coupled resonator unit 140, as in the receive filtercircuit 14 of the first preferred embodiment. The receive filter circuit15 in the second preferred embodiment is different from the receivefilter circuit 14 in the first preferred embodiment in that the otherend of the fourth capacitor C4 is connected to a third path 56 whichconnects the parallel arm resonator P21 and the ground terminal g21. Oneend of the fourth capacitor C4 is connected to the second path 52, asdescribed below.

The second path 52 is connected in parallel with the receive filtercircuit 15 of the first path 50. The second path 52 includes a fifthcapacitor C5, a 2IDT longitudinally coupled resonator 54, and a sixthcapacitor C6 in this order in which they are located closer to the firstcommon terminal 16. The 2IDT longitudinally coupled resonator 54 isgrounded. The left-side IDT electrode of the 2IDT longitudinally coupledresonator 54 is connected to the fifth capacitor C5, while theright-side IDT electrode is connected to the sixth capacitor C6. One endof the fourth capacitor C4 is connected to a path which connects thefifth capacitor C5 and the left-side IDT electrode of the 2IDTlongitudinally coupled resonator 54. The other end of the fourthcapacitor C4 is connected to the third path 56, as described above.

The fourth capacitor C4 may be provided in any suitable manner. Forexample, the fourth capacitor C4 may be provided as a result of thethird path 56 opposing the path which connects the fifth capacitor C5and the 2IDT longitudinally coupled resonator 54.

2.2. Advantages

The isolation characteristics of the filter device 2 according to thesecond preferred embodiment will be discussed below with reference toFIG. 9 . In FIG. 9 , the solid line indicates the isolationcharacteristics of the filter device 2 according to the second preferredembodiment, while the broken line indicates the isolationcharacteristics of a filter device according to a comparative examplenot including the fourth capacitor C4. As shown in FIG. 9 , theattenuation in the transmit band of the filter device 2 of the secondpreferred embodiment is greater than that of the filter device of thecomparative example. That is, the attenuation within the transmit bandin the receive filter circuit 15 is increased due to the fourthcapacitor C4. The reason for this is as follows. By including the fourthcapacitor C4, more parameters are provided to generate a cancel signalin the second path 52. This facilitates the generation of a cancelsignal having the same or substantially the same amplitude and theinverted phase with respect to an unwanted signal.

As well as in the first preferred embodiment, in the second preferredembodiment, it is possible to shift the attenuation pole generated bysub-resonance provided in a radio-frequency band to the lower frequencyside without increasing the size of the filter device 2.

3. Third Preferred Embodiment 3.1. Circuit Diagram of Filter Device

A third preferred embodiment of the present invention will be describedbelow with reference to FIGS. 10 and 11 . FIG. 10 is a circuit diagramof a filter device 3 according to the third preferred embodiment. FIG.11 is a graph illustrating the isolation characteristics of the filterdevice 3.

The filter device 3 of the third preferred embodiment is different fromthe filter device 2 of the second preferred embodiment in that a secondpath 62 is connected in parallel with the transmit filter circuit 13 andthe receive filter circuit 15 in a first path 60. More specifically, thesecond path 62 is connected to a node between the transmit inputterminal Tx and the transmit filter circuit 13 and a node between thereceive output terminal Rx and the longitudinally coupled resonator unit140 of the receive filter circuit 15. The second path 62 includes aneighth capacitor C8, a 2IDT longitudinally coupled resonator 64, and aninth capacitor C9 in this order in which they are located closer to thetransmit input terminal Tx. The 2IDT longitudinally coupled resonator 64is grounded. The left-side IDT electrode of the 2IDT longitudinallycoupled resonator 64 is connected to the eighth capacitor C8, while theright-side IDT electrode is connected to the ninth capacitor C9. One endof a seventh capacitor C7 is connected to a path which connects theninth capacitor C9 and the right-side IDT electrode of the 2IDTlongitudinally coupled resonator 64. The other end of the seventhcapacitor C7 is connected to a third path 66 which connects the parallelarm resonator P21 of the receive filter circuit 15 and the groundterminal g21.

3.2. Advantages

The isolation characteristics of the filter device 3 according to thethird preferred embodiment will be discussed below with reference toFIG. 11 . In FIG. 11 , the solid line indicates the isolationcharacteristics of the filter device 3 according to the third preferredembodiment, while the broken line indicates the isolationcharacteristics of a filter device according to a comparative examplenot including the seventh capacitor C7. As shown in FIG. 11 , theattenuation in the transmit band and that in the receive band of thefilter device 3 of the third preferred embodiment are greater than thoseof the comparative example. That is, the attenuation within the transmitband in the receive filter circuit 15 and that within the receive bandin the transmit filter circuit 13 are increased. The reason for this isas follows. As a result of including the seventh capacitor C7, moreparameters are provided to generate a cancel signal. This facilitatesthe generation of a cancel signal in the second path 62.

As well as in the first preferred embodiment, in the third preferredembodiment, it is possible to shift the attenuation pole generated bysub-resonance formed in a radio-frequency band to the lower frequencyside without increasing the size of the filter device 3.

4.1. Modified Examples

Modified examples of the above-described filter devices will bedescribed below with reference to FIGS. 12 and 13 . FIG. 12 is a circuitdiagram of a filter device 4 according to a first modified example of apreferred embodiment of the present invention. FIG. 13 is a circuitdiagram of a filter device 5 according to a second modified example of apreferred embodiment of the present invention. The filter devices 4 and5 of the first and second modified examples are different from thefilter device 3 of the third preferred embodiment in that theconfiguration of the second path 62 is different.

4.1. Circuit Diagram of Filter Device of First Modified Example

As shown in FIG. 12 , in the filter device 4 of the first modifiedexample, a second path 72 is connected in parallel with the transmitfilter circuit 13 and the receive filter circuit 15 in a first path 70.More specifically, the second path 72 is connected to a node between thetransmit input terminal Tx and the transmit filter circuit 13, the firstcommon terminal 16, and a node between the receive output terminal Rxand the longitudinally coupled resonator unit 140 of the receive filtercircuit 15. In this manner, the second path 72 is connected to the firstpath 70 at three nodes. The second path 72 includes an eleventhcapacitor C11, a twelfth capacitor C12, a thirteenth capacitor C13, anda 3IDT longitudinally coupled resonator 74. The eleventh capacitor C11is connected to a node between the transmit input terminal Tx and thetransmit filter circuit 13. The twelfth capacitor C12 is connected tothe first common terminal 16. The thirteenth capacitor C13 is connectedto a node between the receive output terminal Rx and the longitudinallycoupled resonator unit 140 of the receive filter circuit 15. The 3IDTlongitudinally coupled resonator 74 is connected to the eleventh,twelfth, and thirteenth capacitors C11, C12, and C13.

The 3IDT longitudinally coupled resonator 74 includes three IDTelectrodes. The three IDT electrodes are connected to different groundterminals. The left-side IDT electrode is connected to the eleventhcapacitor C11, the middle-side IDT electrode is connected to the twelfthcapacitor C12, and the right-side IDT electrode is connected to thethirteenth capacitor C13.

The filter device 4 includes a tenth capacitor C10. One end of the tenthcapacitor C10 is connected to the second path 72, while the other end ofthe tenth capacitor C10 is connected to a third path 76 which connectsthe parallel arm resonator P21 and the ground terminal g21. Morespecifically, one end of the tenth capacitor C10 is connected to a nodebetween the thirteenth capacitor C13 and the right-side IDT electrode ofthe 3IDT longitudinally coupled resonator 74, while the other end of thetenth capacitor 10 is connected to the third path 76.

4.2. Circuit Diagram of Filter Device of Second Modified Example

As shown in FIG. 13 , as in the first modified example, in the filterdevice 5 of the second modified example, a second path 82 is connectedto a first path 80 at a node between the transmit input terminal Tx andthe transmit filter circuit 13, the first common terminal 16, and a nodebetween the receive output terminal Rx and the longitudinally coupledresonator unit 140 of the receive filter circuit 15. The second path 82includes a sixteenth capacitor C16, a seventeenth capacitor C17, aneighteenth capacitor C18, and a 2IDT longitudinally coupled resonator84. The sixteenth capacitor C16 is connected to a node between thetransmit input terminal Tx and the transmit filter circuit 13. Theseventeenth capacitor C17 is connected to the first common terminal 16.The eighteenth capacitor C18 is connected to a node between the receiveoutput terminal Rx and the longitudinally coupled resonator unit 140 ofthe receive filter circuit 15. The 2IDT longitudinally coupled resonator84 is connected to the sixteenth, seventeenth, and eighteenth capacitorsC16, C17, and C18.

The 2IDT longitudinally coupled resonator 84 includes two IDTelectrodes. The left-side IDT electrode is connected to the sixteenthand seventeenth capacitors C16 and C17, while the right-side IDTelectrode is connected to the eighteenth capacitor C18.

The filter device 5 includes a fifteenth capacitor C15. One end of thefifteenth capacitor C15 is connected to the second path 82, while theother end of the fifteenth capacitor C15 is connected to a third path 86which connects the parallel arm resonator P21 and the ground terminalg21. More specifically, one end of the fifteenth capacitor C15 isconnected a node between the eighteenth capacitor C18 and the right-sideIDT electrode of the 2IDT longitudinally coupled resonator 84, while theother end of the fifteenth capacitor C15 is connected to the third path86.

4.3. Advantages

As in the filter device 3 according to the third preferred embodiment,in the filter devices 4 and 5 according to the first and second modifiedexamples, the attenuation within the transmit band in the receive filtercircuit 15 and that within the receive band in the transmit filtercircuit 13 are increased. As well as in the first preferred embodiment,in the first and second modified examples, it is possible to shift theattenuation pole generated by sub-resonance formed in a radio-frequencyband to the lower frequency side without increasing the sizes of thefilter devices 4 and 5.

The above-described preferred embodiments are provided to facilitate theunderstanding of the present invention, but are not intended to beexhaustive or to limit the present invention to the precise structureand configurations disclosed. The elements of the preferred embodimentsand the positions, materials, conditions, configurations, and sizesthereof are not restricted to those described in the preferredembodiments and may be changed in an appropriate manner. The elements ofthe different preferred embodiments may be partially replaced by orcombined with each other.

For example, although the second path is connected to the first path attwo or three nodes in the above-described preferred embodiments, it maybe connected to the first path at four or more nodes.

In the above-described preferred embodiments, the capacitor connected tothe second and third paths is provided therebetween as a result of thesecond and third paths opposing each other. However, a differentapproach may be taken to provide such a capacitor. For example, in thefirst preferred embodiment, the first capacitor C1 may be provided as aresult of the second path 20 and the parallel arm resonator P11 opposingeach other. More specifically, the first capacitor C1 may be provided asa result of the second path 20 and a comb-shaped electrode or a busbar(neither of them is shown) of the parallel arm resonator P11 opposingeach other. The busbar opposing the second path 20 may be a busbarconnected to the ground terminal g11. Similarly, the fourth capacitorC4, the seventh capacitor C7, the tenth capacitor C10, and the fifteenthcapacitor C15 may be provided in a manner other than those described inthe corresponding preferred embodiments.

The capacitor connected between the second and third paths may beconnected to any one of the parallel arm resonators of the transmitfilter circuit or the receive filter circuit. For example, in the firstpreferred embodiment, the first capacitor C1 is connected to the thirdpath 24 connecting the parallel arm resonator P11 and the groundterminal g11. However, instead of to the third path 24, the firstcapacitor C1 may be connected to the path which connects the parallelarm resonator P12 and the ground terminal g11 or the path which connectsthe parallel arm resonator P13 and the ground terminal g12.

Although the grounded resonator disposed on the second path is a 2IDT or3IDT longitudinally coupled resonator in the above-described preferredembodiments, it may be a longitudinally coupled resonator including fouror more IDT electrodes. In accordance with the number of IDT electrodesof the longitudinally coupled resonator, the number of nodes on thefirst path that the second path is connected can be determined.

In the above-described preferred embodiments, each of the filter devicesis applied to a duplexer. However, a filter device according to apreferred embodiment of the present invention may be applicable tovarious filter devices including the corresponding number of filtercircuits. For example, the filter device may include a single filtercircuit. The filter device may be a duplexer including a composite oftwo filter circuits, as explained in the above-described preferredembodiments, a triplexer including a composite of three filter circuits,a quadplexer including a composite of four filter circuits, and anoctaplexer including a composite of eight filter circuits.

In the first preferred embodiment, the ceramic layer is disposed as theintermediate layer 44 between the first wiring 43 and the second wiring45. However, the ceramic layer may be omitted.

In the above-described preferred embodiments, the resonator disposed onthe second path is a longitudinally coupled resonator. However, theresonator disposed on the second path may be another type of resonator,such as a resonator used as a parallel arm resonator in theabove-described preferred embodiments.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A multiplexer comprising: a first terminal; a second terminal; athird terminal; a ground terminal; a first path including at least onefirst series arm resonator, a first parallel arm resonator, and at leastone second series arm resonator; a second path including a groundedresonator; a third path connecting the first parallel arm resonator tothe ground terminal; and a capacitor; wherein a first end of the firstpath is connected to the first terminal, and a second end of the firstpath is connected to the second terminal; the third terminal isconnected to the first path between the first and second ends of thefirst path; the at least one first series arm resonator and the firstparallel arm resonator are disposed between the first and thirdterminals; the at least one second series arm resonator is disposedbetween the second and third terminals; the second path is connected inparallel with a portion of the first path such that a first end of thesecond path is connected to the first path between the first and thirdterminals and a second end of the second path is connected to the firstpath between the second and third terminals; and one end of thecapacitor is connected to the second path, and another end of thecapacitor is connected to the third path.
 2. The multiplexer accordingto claim 1, wherein the grounded resonator is a longitudinally coupledresonator.
 3. The multiplexer according to claim 1, wherein thecapacitor is defined by the second path and the third path opposing eachother.
 4. The multiplexer according to claim 3, wherein a ceramic layeris disposed in at least a portion of a space sandwiched between thesecond path and the third path where the capacitor is provided.
 5. Themultiplexer according to claim 1, wherein the first path includes atransmit filter circuit and a receive filter circuit; and one of thetransmit and receive filter circuits includes the at least one firstseries arm resonator and the first parallel arm resonator, and the otherof the transmit and receive filter circuits includes the at least onesecond series arm resonator.
 6. The multiplexer according to claim 5,wherein the second path is connected in parallel with at least a portionof the transmit and receive filter circuits.
 7. A multiplexercomprising: a first terminal; a second terminal; a third terminal; aground terminal; a first path including at least one first series armresonator, a first parallel arm resonator, and at least one secondseries arm resonator; a second path including a grounded resonator; anda third path connecting the first parallel arm resonator to the groundterminal; wherein a first end of the first path is connected to thefirst terminal, and a second end of the first path is connected to thesecond terminal; the third terminal is connected to the first pathbetween the first and second ends of the first path; the at least onefirst series arm resonator and the first parallel arm resonator aredisposed between the first and third terminals; the at least one secondseries arm resonator is disposed between the second and third terminals;the second path is connected in parallel with a portion of the firstpath such that a first end of the second path is connected to the firstpath between the first and third terminals and a second end of thesecond path is connected to the first path between the second and thirdterminals; and a first wiring defining the second path and a secondwiring defining the third path oppose each other.
 8. The multiplexeraccording to claim 7, wherein an insulator is disposed between the firstwiring and the second wiring.
 9. The multiplexer according to claim 7,wherein the grounded resonator is a longitudinally coupled resonator.10. The multiplexer according to claim 7, wherein the first pathincludes a transmit filter circuit and a receive filter circuit; and oneof the transmit and receive filter circuits includes the at least onefirst series arm resonator and the first parallel arm resonator, and theother of the transmit and receive filters includes the at least onesecond series arm resonator.
 11. The multiplexer according to claim 10,wherein the second path is connected in parallel with at least a portionof the transmit and receive filter circuits.
 12. The multiplexeraccording to claim 5, wherein the at least one first series armresonator includes a plurality of first series arm resonators; and theat least one second series arm resonator includes a plurality of secondseries arm resonators.
 13. The multiplexer according to claim 10,wherein the transmit filter circuit and the receive filter circuitdefine surface acoustic wave filters.
 14. (canceled)
 15. The multiplexeraccording to claim 5, wherein the transmit filter circuit and thereceive filter circuit define surface acoustic wave filters. 16-19.(canceled)
 20. The multiplexer according to claim 1, further comprisinga piezoelectric substrate on which the at least one first series armresonator, the first parallel arm resonator, the at least one secondseries arm resonator, and the capacitor are provided.
 21. Themultiplexer according to claim 7, further comprising a piezoelectricsubstrate on which the at least one first series arm resonator, thefirst parallel arm resonator, the at least one second series armresonator, and the first and second wirings are provided.
 22. Themultiplexer according to claim 5, wherein the other of the transmit andreceive filter circuits further includes a second parallel armresonator.
 23. The multiplexer according to claim 10, wherein the otherof the transmit and receive filter circuits further includes a secondparallel arm resonator.